Identifying the presence of hydrocarbons, such as oil and natural gas, relies on using subsurface data to detect hydrocarbon systems. In order to extract hydrocarbon accumulations associated with such a system, Earth scientists typically decide where to place and drill wells (injectors and producers), how to ensure consistent fluid flow into a well, and how to reduce risks.
One set of deposits encountered during the drilling process are a group of compounds known as clathrates, which are frozen gas accumulations typically existing in the shallow subsurface. Clathrate deposits were conventionally considered a drilling hazard, but are now also being thought of as a potential seal and resource.
Current estimates of methane sequestered globally in clathrates is between 100,000 and 5,000,000 TCF with the most frequently quoted estimate of 700,000 TCF (a number which excludes any hydrates located in Antarctic or alpine permafrost areas). Even the lowest estimate represents an enormous potential new energy resource, equal to more than 4,000 times the amount of natural gas consumed in the US or 18 times the entire world's proven gas resources.
Clathrates are substances in which a lattice structure made up of one molecular component (host molecules) traps or encases another molecular component (guest molecules); they resemble crystal-like structures. Clathrates can be found in relatively low temperature and relatively high pressure environments in deep-water (ocean) and permafrost (terrestrial) areas within the clathrate stability zone. Large quantities of hydrocarbon gas are closely packed together by this mechanism. For example, a cubic meter of natural gas hydrate contains approximately 0.8 cubic meters of water and generally 164 cubic meters of natural gas at standard (surface) temperature and pressure conditions.
The thickness of the clathrate stability zone varies with temperature, pressure, composition, and availability of the hydrate-forming gas, underlying geologic conditions, water depth, salinity, and other factors. The clathrate stability zone is an interval with a top and base. Within the zone, if hydrocarbons are present, they generally occur in a frozen state. Below the clathrate stability zone, increased pressure and temperature force hydrocarbons into a gaseous state. The top of the zone is regularly defined as the sea floor (transition from ocean water to sediment).
The current state of the art for identifying clathrate deposits from seismic data relies on using a bottom simulating reflector (BSR) as a proxy for the base of the clathrate stability zone (FIG. 1A). The BSR is thought to delineate an abrupt phase change (e.g., solid to gas) from frozen gas (clathrates) above to free gas below it. One disadvantage of using the BSR to delineate the base of the clathrate stability zone is that the BSR is often present where no clathrate deposits exist, and vice versa (FIG. 1B).
Furthermore, the current state of the art for quantifying clathrate deposits relies on either modeling an estimated clathrate volume and saturation, or on taking direct measurements via drilling at least one well. Both techniques are regularly performed in one dimension (1D), and therefore do not account for the three dimensional (3D) volume of the clathrate deposits themselves.
As such, an improved manner of identifying clathrate deposits is needed.